Oral Roberts, who died on Tuesday aged 91, was the doyen of American
televangelists, a breed that has played a vital part in guiding a third of
adult Americans to a life as born-again Christians.

From the time he began his ministry in the 1940s Roberts was one of the most successful practitioners of the art of blending a hellfire vision with the growing material success of his traditionally poor and unsophisticated followers. His creed dictated that people are saved not so much by grace but by the uses to which they put their money. His favourite tele-blessing ran: "May God bless you in your bodies, in your spirits and in your finances."

One of the first of a generation of Protestant preachers who came to dominate the airwaves in America, Roberts placed his miraculous powers of faith healing at the centre of his ministry. In his early days, when he addressed gatherings of the faithful in tents, he would lay his (divinely anointed) right hand on the afflicted and claimed to achieve instant cures. Petitioners who failed to respond to his ministrations were bawled out.

As he moved on to the airwaves, Roberts routinely attempted to heal his followers of a wide range of diseases – from cancer to haemorrhoids – by placing his healing hand against the lens of the camera and asking the afflicted to touch their television screens. The miracle cures thus effected were often accompanied by requests for "seed faith" donations – "twenty dollars, Visa, Amex, whatever the Lord leads you to do".

By the early 1980s the Oral Roberts Evangelistic Association employed 2,300 people and was grossing $110 million a year in donations. At its base in Tulsa, Oklahoma, Roberts built a sort of divine Disneyworld, graced by the world's largest bronze sculpture, a 60ft pair of hands clasped in prayer, a 200ft glass and steel "prayer tower" topped by an eternal gas flame, and a 777ft artificial stream called the River of Life.

There was also an Oral Roberts University, opened in 1965, where students were exhorted to dress and behave in an attractive way. Women were encouraged to wear make-up and fashionable clothes. The chronically fat risked expulsion.

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Besides the university, there were a television studio, a radio station, some skyscrapers and a concert hall. A monthly magazine called Abundant Life gained a circulation of 1.2 million. About 800,000 of Roberts's most loyal followers received monthly prayer letters. "Are you suffering?" ran a typical missive. "Is pain raging through your body? Is your marriage falling apart? Does it seem that nobody cares? I want to help you get the answer you need. I want to pray with you."

Each day Roberts would get a thick typed list of the names and needs of those who had written in with pleas for help, and prayed for those with the most serious afflictions from the top of his prayer tower. Anyone needing urgent assistance could call prayer counsellors at the tower, who were on duty 24 hours a day to give spiritual comfort.

In the late 1970s, inspired by a "conversation" with God, Roberts embarked on his most ambitious project – a City of Faith medical and research complex consisting of a 60-storey clinic and a hospital with 777 beds (according to Roberts, 777 was a mystical number). Although the Tulsa authorities told him there was no need for the hospital, Roberts went ahead anyway, drawing comfort from a vision of a 900ft-tall figure of Christ which picked up the enormous building in his hands and said: "Look how easy it is."

But the secular authorities turned out to be right. By the mid-1980s only a few score of the beds had been filled and the hospital was running a million-dollar-a-month deficit. In a dramatic announcement in July 1984, Roberts reported that Jesus had once again visited him, and an "angel of the Lord" had been placed at his disposal. The angel was dispatched to bring the "poor, needy and the sick" to the City of Faith, but it seemed that most preferred conventional medicine. By the end of 1986 the financial situation was desperate.

In January 1987 Roberts told his followers that he had been having an "ongoing conversation" with God, who had threatened to "call him home" unless they stumped up $8 million by March 31 for his medical missionary programme. His son Richard followed this up by sending a birthday card to supporters, asking them to return to it to his father with a cheque: "Let's not let this be my Dad's last birthday," he wrote. At the end of January his followers were relieved to hear that Roberts had been granted a reprieve, as God had extended the deadline to the end of the year.

Sad to say, Roberts's gambit was not treated with reverence in all quarters. Las Vegas bookmakers gave odds about whether or not he would make it; an outfit called Lord (Let Oral Roberts Die) attracted followers. A Tulsa disc jockey announced he had seen a 600ft vision of Lassie which told him, "If you don't send me money, you will die." Roberts did not see the funny side, and the radio station lost $8,000 in advertising.

But the tactic worked – in the short term. Roberts raised $9.1 million, including a donation of $1.3 million from a dog track owner, and eventually descended from his prayer tower, where he had been holding a fast. The following week the tower was struck by lightning. No one was hurt.

More ominously for Roberts, though, some television stations had refused to broadcast his appeal, objecting that it smacked of extortion. Fellow evangelists attacked his tactics as "bad for the gospel and bad for business". By 1989 his financial difficulties had got the better of him and he was forced to close the medical centre. The affair rather knocked the stuffing out of Roberts's ministry, though he remained chancellor of Oral Roberts University.

Oral Granville Roberts was born on January 24 1918 in Pontotoc County, Oklahoma, the fifth and youngest child of a "sawdust trail" preacher on the fringes of the Pentecostal movement. In The Call (1976), his folksy autobiography (described by one reviewer as "Pilgrim's Progress as it might have been written by Horatio Alger"), Roberts described a household in which "I felt quite sure that Jesus lived with us because Mama and Papa talked to Him so much."

His parents expected their youngest son to follow in his father's footsteps. On one occasion his mother vowed to give her child to God in return for the healing of a neighbour's offspring. Roberts received his own calling after "miraculously" recovering from tuberculosis. God told him: "Son, I'm going to heal you and you are going to take the message of My healing power to your generation."

After leaving high school, his formal education consisted of the equivalent of about two years of college study, all taken part-time at Bible schools in Oklahoma. He duly joined his father on the sawdust trail, preaching in tents and at outdoor camps. In 1938 he married Evelyn Lutman Fahnestock, also the daughter of a Pentecostal preacher, whom he met while both were playing guitars and singing at a camp meeting.

For the first nine years of their marriage, the Robertses subscribed to the doctrine "blessed are the poor", yet they secretly longed to escape the poverty into which they had been born. Roberts toured the evangelical circuit and held four brief pastorates before he found justification for their suppressed desires in the biblical passage: "I wish above all things that thou mayest prosper and be in health, even as thy soul prospereth."

Roberts considered it a revelation, and immediately went out and bought his first new car, a Buick, which he regarded as "a symbol of what a man could do if he would believe in God". A month later he was told by God to concentrate his ministry on healing: "You will have the power to pray for the sick and cast out devils."

In April 1947 he began holding healing services in his church at Enid, Oklahoma, on Sunday afternoons. Word spread, and by May the crowds were so large that he rented a larger building. In June he announced to his congregation that he had received invitations to conduct healing services in eight states and had decided to resign his pastorate.

Within two years his radio and television programmes were being broadcast on hundreds of stations in the United States, Canada, Alaska and Hawaii and by short-wave to listeners around the world. By 1957 his evangelistic association claimed he was reaching an audience of nearly a billion.

For two decades Roberts conducted his crusades as a minister of the Pentecostal Holiness Church, but he felt increasingly cramped by its theology and prudishness, which might have hindered his televisual performances. In 1968 he was accepted as a United Methodist minister.

Roberts became a pillar of the Tulsa community, a director of its chamber of commerce and of one of its largest banks. He remained acutely sensitive about press criticism of his lifestyle, instructing his aides to airbrush the diamond rings he habitually wore out of publicity photographs.

But in 1979 a book published by Jerry Sholes, a former employee, gave a vivid description of the Roberts lifestyle: "He dresses in Brioni suits that cost $500 to $1,000; walks in $100 shoes; lives in a $250,000 house in Tulsa and has a million-dollar home in Palm Springs; wears diamond rings and solid gold bracelets ... drives $25,000 automobiles which are replaced every six months; flies around the country in a $2 million Fanjet Falcon ... and plays games of financial hanky-panky that have made him and his family members independently wealthy (millionaires) for life."

In 1987 Roberts was one of the few to come to the aid of disgraced televangelist Jim Bakker, who had become involved in a sordid sex and financial scandal. "I sat with Bakker and he told me 'Yes, seven years ago, I sinned. I'm not guilty but I did wrong'," Roberts told his followers. Roberts then said he asked Bakker: "What did you do?" – at which point a thunderstorm over Tulsa cut off the television signal.

Roberts's last three decades were marked by tragedy. His daughter Rebecca and her husband were killed in an air crash in 1977. Five years later his rebellious elder son Ronnie committed suicide. A grandchild also died.

Oral Roberts's wife, Evelyn, died in 2005, and he is survived by a son and a daughter. In 1987 he told his followers that he confidently expected to return one day to rule alongside Jesus Christ.